1. Field of the Invention
The present invention relates to an image processing system capable of correcting image imperfections to increase image quality.
2. Description of the Related Art
In the art of image processing system, several techniques are known to improve jaggies that occur on images printed with printers with a low resolution.
Jaggies are jagged edges around printed characters, that is, they are stairstep-like irregularities that appear at edges of characters. An example is shown in FIG. 21.
FIG. 21 is a schematic diagram showing a manner in which a character is processed by a conventional image processing system. In the example shown in FIG. 21, jaggies occur at edges of the character.
In FIG. 21, each of small blocks arranged in a matrix form corresponds to one of pixels arranged at a density of, for example, 300 dpi. In the specific example shown in FIG. 21, a Japanese syllabary character (hiragana) is displayed with a resolution of 300 dpi, and stairstep-shaped jaggies appear in curved portions.
It is known in the art to detect portions having jaggies by means of pattern matching and then adding image data to the detected portions thereby reducing jaggies. Such a process for reducing jaggies is also called a smoothing process.
Pattern matching is a well known technique and a description thereof may be found, for example, in Japanese Patent Laid-Open No. 10-42141, and thus details of the pattern matching technique are not described herein.
FIGS. 22A and 22B are schematic diagrams showing examples of characters processed by a conventional image processing system. In those examples, jaggies at edges or in other portions of characters are improved.
In the example shown in FIG. 22A, halftone data is added to pixels detected in pattern matching. More specifically, halftone data is added to pixels forming stairsteps shown in FIG. 21. In particular, in printers based on electrophotographic process, a good improvement is achieved simply by adding halftone data to portions having jaggies.
In the example shown in FIG. 22B, pixels in particular areas detected by pattern matching are each divided into sub-pixels, and data is added at sub-pixel positions.
Dividing of pixels is performed such that the pixels are divided into sub-pixels, and dots are formed at some sub-pixel positions. In the example shown in FIG. 22B, each pixel is divided into two sub-pixels. A subdivision technique is also disclosed in Japanese Patent Laid-Open No. 10-42141, and thus details thereof are not described herein.
In the smoothing process described above, half dots or sub-dots are added to portions having jaggies. Conversely, some full dots may be converted into half dots or some full dots may be divided into sub-pixels so as to reduce the pixel size in portions having jaggies.
A technique of easing the above problem in the image processing system is disclosed in Japanese Patent Laid-Open No. 10-42141 ('141 patent).
However, generally, conventional systems have various disadvantages as described below with reference to FIGS. 23A and 23B.
FIGS. 23A and 23B are schematic diagrams showing examples of characters processed by the conventional image processing system. In those examples, stairstep-shaped jaggies at edges or in other portions of characters are improved.
If a halftone character shown in FIG. 23A is subjected to an image forming process such as screen processing, the character line can become discontinuous as shown in FIG. 23B.
This occurs when halftone characters are represented with a screen resolution lower than a printer resolution such as 300 dpi.
In this case, smoothing based on pattern matching cannot remove the jaggies.
The screen resolution cannot be greater than the printer resolution, because the screen resolution is represented in a pseudo fashion by combining a plurality of pixels (dots) of a printer into a single pixel. For example, the screen resolution (number of lines) generally ranges from 133 to 175 lines. Even in a very special case, the resolution is limited to 268 lines. The screen resolution is limited to the above described range because higher screen resolutions do not lead to high quality in images formed by electrophotographic printers.
To give a better understanding of the structure of jaggies, FIG. 24 shows a macroscopic view of a part of the character shown in FIG. 23B.
In FIG. 24, character processing by a conventional image processing system is schematically shown. In the example shown in FIG. 24, jaggies appear at edges of the character.
Jaggies discussed herein appear at edges when image data is subjected to screen processing.
In addition to jaggies that appear at edges due to the above-described printer resolution, improvement of jaggies that appear due to low resolutions in screen processing will also be discussed herein.
When an image compressed by means of lossy compression such as JPEG compression is subjected to smoothing process, mosquito noise (that occurs in an image with high-frequency components or occurs at edges of an image when the data size is compressed by a large factor by means of lossy compression) can occur due to degradation caused by the compression. The mosquito noise can prevent the matching process from detecting a jagged edge. If such a failure occurs in the matching process, good smoothing cannot be achieved.
Although the '141 patent attempts to address some of the above disadvantages, it has been found that many conventional image processing systems are unable to provide suitable smoothing for both multilevel images and for images based on processes other than error diffusion process.